In x-ray diagnostics, use is increasingly made of electronic image detectors. While, in particular, individual recordings are still frequently recorded with x-ray sensitive film material, even nowadays, it is desirable to use electronic image detectors, above all in the case of series of x-ray recordings. They permit, firstly, the recording of faster image sequences on account of the lower requisite exposure times and dispensing with the mechanical film transport, and, secondly, the data registered can be supplied immediately for electronic evaluation. For example, in computer tomography, a series of thousands of individual recordings have to be assembled to form an overall image, for example to form an x-ray slice.
In computer tomography, use is made of image detectors which include individual module boards. These are in each case composed of a scintillator and an adjacent row of photosensors, which is structured in individually contacted detection points, that is to say in pixel points. The photosensor layer includes a photodiode material and, for each pixel point, that is to say on each measurement channel, supplies an analog current which is proportional to the measured x-ray intensity. An A/D converter converts this into a digital value which is supplied to the evaluation computer as an individual image point, that is to say as a pixel.
Since each pixel point of the photodiode supplies its own measured value via an associated measurement channel, a dedicated electric line must be provided to make contact with each pixel point. Hitherto, the pixel points were arranged linearly, that is to say in a row. Contact is produced by contact surfaces which are arranged in a parallel row arranged separately from the pixel points and to which electric conductor tracks lead. Each contact surface serves as a bonding pad for the respective electric line. The electric lines are led laterally out of the elongate photodiode.
In computer tomography, use is made of module boards having the largest possible number of measurement channels; 10 000 measurement channels and more are desirable. The bonding techniques used are becoming increasingly difficult to handle with such large channel numbers. In addition, one is limited by the bonding technique to a linear arrangement of the photodiodes or pixel points, since, in a flat arrangement, the bonding pads would assume an excessively large surface, which would be viewed as a dead zone for the detector.
A further disadvantage of the previous module construction resides in the long signal paths which were produced between the linearly arranged pixel points and the evaluation electronics. The long signal paths firstly cause long signal propagation times, which are disadvantageous for the evaluation of series of images which follow one another quickly, and secondly they are susceptible to interference signals. Not least, the cost for the production of such a large number of bonding connections is very high. In view of the difficulties of the previous technology, an increase in the number of channels to more than 10 000 appears not to be practical.
Although flat detector concepts are also known instead of the module structure and achieve a higher level of integration in a more cost-effective manner, these concepts, for example panels of amorphous silicon, cannot readily be implemented in computer tomography. In particular, the persistence of the previously known detectors is responsible for the fact that no adequate time resolution and recording dynamics have been achieved.